<!DOCTYPE HTML>
<html>
<head>
<style>
div:after { content: 'X'; border:1px solid red; }
div.foo:after { content: 'Y'; }
</style>
</head>
<body>
<div id="d"></div>
<script>
document.body.offsetTop;
document.getElementById("d").setAttribute("class", "foo");
</script>
</body>
</html>
